Regarding heat-sensitive lithographic printing plate precursors, which can be mounted on a printing machine without being subjected to any processing to conduct printing, various methods have been proposed. One promising method is a method utilizing ablation, which comprises exposing a lithographic printing plate precursor to a high output solid infrared laser, for example, semiconductor laser or YAG laser and generating heat in the exposed area by a light-heat converting agent that converts light to heat to cause destructive evaporation.
Namely, the method comprises providing a hydrophilic layer on a substrate having a lipophilic ink-receptive surface or a lipophilic ink-receiving layer and eliminating the hydrophilic layer by ablation.
In WO94/18005 is disclosed a printing plate comprising a crosslinked hydrophilic layer on a lipophilic laser beam-absorbing layer, wherein the hydrophilic layer is subjected to ablation. The hydrophilic layer comprises polyvinyl alcohol crosslinked with a hydrolyzate of tetraethoxysilicon and particulate titanium dioxide so as to improve film strength of the hydrophilic layer. According to the technique, press life can be improved. However, since the hydrophilic layer is mainly composed of the polyvinyl alcohol having hydrocarbon groups, which is not necessarily highly hydrophilic, it is insufficient in stain resistance. Thus, further improvements have been required in the hydrophilic layer.
In WO98/40212, WO99/19143 and WO99/19144 are disclosed lithographic printing plate precursors capable of being mounted on a printing machine without development, which comprise a substrate provided thereon, in order, an ink-receiving layer and a hydrophilic layer mainly composed of colloid, for example, silica, crosslinked with a crosslinking agent such as aminopropyltriethoxysilane. In the hydrophilic layer, it is attempted that resistance to printing stain is enhanced by reducing the amount of hydrocarbon group as small as possible and press life is improved by crosslinking the colloid with a crosslinking agent.
However, according to the above-described technique, press life is insufficient as several thousands of sheets. Hitherto known digital direct process-less printing plates utilizing ablation had a problem of being deteriorated in either stain resistance or press life, which are essential requirements in printing, because of difficulties in realizing the process-less technique.
It is describe in Japanese Patent Laid-Open No. 96936/2001 that a heat-sensitive lithographic printing plate precursor comprising a substrate having an ink-receptive surface or an ink-receiving layer coated thereon a three dimensionally crosslinked hydrophilic layer and a water-soluble overcoat layer in this order, wherein the hydrophilic layer comprises a colloid of an oxide or hydroxide of at least one element selected from a group consisting of beryllium, magnesium, aluminum, silicon, titanium, boron, germanium, tin, zirconium, iron, vanadium, antimony and a transition metal, and a hydrophilic resin and the heated area of the hydrophilic layer is capable of being readily eliminated with dampening water or ink in at the time of printing allows the compatibility between press life and stain resistance in a digital direct process-less printing plate. However, it has been found that further improvements in ink receptivity at the beginning of printing in order to reduce spoilage and sensitivity are required even in such a heat-sensitive lithographic printing plate precursor.